1. Field of the Invention (Technical Field)
The present invention relates to a scanning x-ray apparatus operating with a peak x-ray energy of up to or greater than 1 MeV; and a method of using such apparatus.
2. Background Art
X-ray transmission images are widely used in modern technology including their use in medical imaging, non-destructive testing of industrial equipment, and for the inspection of cargo and baggage for contraband.
X-ray images--both transmission and scattered--are normally made from x-ray sources, which are fixed in space and as small as possible in area. The x-rays are produced by the impact of a beam of electrons on a target. This necessitates high power density of electrons on the target. The spatial resolution and contrast of x-ray images are dependent upon the size of the x-ray source, the x-ray signal level and the x-ray signal-to-noise ratio.
Further, as set forth in "Varian Linatron--High Energy X-Ray Applications for Nondestructive Testing", pp 26-27 (no date given on publication), Varian Associates, Palo Alto, Calif., image quality requires sharpness and high contrast to assure good radiographic quality. The primary sources of unsharpness in radiography are inherent unsharpness, U.sub.i, and geometric unsharpness, U.sub.g, which is defined by the expression: EQU U.sub.g =S/{D/T}, (1)
where
S=source spot diameter, PA1 D=distance from the source spot to the object, PA1 T=thickness of the object. PA1 Thus, for P=1 kW, the minimum source diameter, S, is 1 mm; for P=4 kW the S must be at least 4 mm. The reason for this limitation is the fundamental heat transfer capability of high temperature materials as described in "Energy-Beam Processing of Materials", Taniguchi, Clarendon Press, Oxford, pp 19-21, 1989. Equipment which intrinsically embodies this power limitation in conjunction with it's means of producing an X-ray pencil beam and X-ray image, is described in J. A. Stein and R. Swift, U.S. Pat. No. 3,780,291. In that device, the X-rays are produced by an X-Ray tube in which the flux of X-rays is limited by the heating of the anode of the X-Ray tube. PA1 Z=the thickness of the material penetrated (g/cm.sup.2) normalized to the density PA1 .mu.=the mass attenuation coefficient of the penetrated material (cm.sup.2 /g). PA1 1) the power of the accelerator, PA1 2) the maximum amount of electron power which can be incident on the target without melting it, and PA1 3) the distance between the x-ray source location and the material being penetrated.
Source spot size is thus critical to image quality. Heating which can lead to melting of the target limits the ratio of source spot diameter, S, to the power, P, to a practical value of EQU P/S&lt;1 kw/mm. (2)
One prior art device used in overcoming this limitation is to move the x-ray target material rapidly in front of the electron beam while keeping the electron beam in a fixed location, as in the well-known "rotating anode" x-ray source. Another prior art embodied in U.S. Pat. No. 4,521,900 to Rand, is used for tomographic scanning at lower energies of the electron beam and moves the electron beam by deflection.
No prior art, however, suggests using a moving x-ray beam in conjunction with a stationary collimator to produce a moving pencil beam of x-rays, as described in the current invention. The moving pencil beam of x-rays is then used to produce an x-ray image. The moving pencil beam of x-rays is produced by changing the trajectory of an electron beam magnetically such that the intersection of the electron beam with the target material moves in a pre-selected manner across the target material. The motion of the beam may be in a manner similar to that suggested in U.S. Pat. No. 4,281,251 to Cleland and Thompson.
The significance of X-ray flux to X-ray imaging is elucidated by understanding the limits of X-ray imaging of thick materials. The fundamental limit to x-ray imaging through thick materials is determined by the number of photons which exit the material and are detected by the detector, and the ratio of this number to the number of "background" or equivalent noise photons produced in the detector in the same time interval. As a rule-of-thumb, the number of photons required to image a pixel (the minimum area detected in the x-ray image) is 5 times background. If there is much less than 1 background photon/pixel, 5 photons per pixel are required to make a good image.
The transmitted photon flux, F.sub.t, in a "good" geometry system (one which is not limited by scattered radiation) such as that defined by pencil beam imaging is given by: EQU F.sub.t =F.sub.0 *exp(-Z*.mu.), (3)
where
If F.sub.tmin is the minimum detectable photon flux, the maximum thickness of material which can be penetrated is: EQU Z.sub.wax =[In(F.sub.0 /F.sub.tmin)]/.mu. (4)
Several factors determine F.sub.0 including,
F.sub.0 is proportional to the electron beam power for fixed beam energy, and F.sub.0 increases as (beam energy).sup.(1,8), for fixed beam power. .mu. varies slowly with x-ray photon energy, slowly decreasing with higher energies. The value of F.sub.o also varies inversely with distance squared from the point source. The valuable result is a technique which allows one to place the source closer to the object. Because high energy x-ray sources produce a relatively small angle cone of emission, the distance from a point x-ray source to the object must be greater than the projected dimension of the object in a conventional prior art point source x-ray system such as the system described in the '291 patent of Stein and Swift. In the present invention, using a high energy electrostatic generator and particle accelerator such as that disclosed in U.S. Pat. No. 5,124,658, to Adler, entitled Nested High Voltage Generator/Particle Accelerator, the electron beam intersects the target along a line (by differing trajectories) reducing the average power per unit area on the x-ray target by a factor of more than 100 for a typical object with a projected dimension of 10'. Thus F.sub.0 may be increased by a factor of 100 without melting the target. Using equation 4, assuming water as the absorbing material, and an average x-ray photon energy of 1 MeV, the penetration, Z, may be increased by 70 cm (27") without melting the target.
Alternatively where fine inspections are required, the x-ray spot size may be reduced without a dramatic decrease in power of the x-ray source which would take place in conventional X-Ray tubes.
The ability to move the x-ray source close to the object results from the fact that the collimator does not need to converge on a point source of X-ray radiation. Instead, the source becomes a line of radiation which is unique to the present invention, compared to prior art "point" source systems. This property leads to an increase of incident photon flux, F.sub.0, of 4 to 40 times, thereby enabling an increase of penetration of 1.4/.mu. to 3.7/.mu.. Using the same example as above, corresponds to an increase of penetration through an object consisting mainly of water of 21 cm to 55 cm (8" to 22").
Because the electron beam has no mechanical inertia, in contrast to the prior art which uses mechanically moving collimators, it is possible to instantaneously speed up, slow down, or discontinuously move the beam from one point on the target line to another one. This feature allows different portions of the object with widely different thickness to receive different incident x-ray fluxes. This "Smart Imaging" can be accomplished automatically by using the signal recorded in the detector in a feedback loop to the deflecting coils which determine the position of the electron beam. In one embodiment, "Smart Imaging" can be used to set the level of measured photon flux to a fixed level with the desired signal to noise level, and the X-ray beam will dwell at each location long enough to generate the fixed level of photon flux before moving on. In this embodiment the attenuation is inversely proportional to the dwell time. An attractive feature of this embodiment is that the stray radiation generated is the minimum possible for a given signal to noise ratio and photon flux. A second attractive feature of this embodiment is that the data recording system can be greatly simplified. For example, a 4 bit digitizer or an analog comparator supplemented by a single time measurement device could be used to replace a 16 bit digitizer. The lateral motion and additional scans are in general produced by moving the object to be imaged by a stationary MeVScan device with the MeVScan device producing a line of the image. Sequential lines of the image are produced by repetitive scans of the object as moved by a conveyor, or the like, and taken at sequential times.